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X-Rays, IR Light Pulses and Laser Cooling

Photonics.comMay 2010
WASHINGTON, DC, May 17, 2010 — Researchers from around the world will present the latest breakthroughs in electro-optics, innovative developments in laser science, and commercial applications in photonics at the 2010 Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference (CLEO/QELS) May 16 to 21 at the San Jose McEnery Convention Center in San Jose, Calif.

The Stanford Linear Accelerator Center (SLAC), long the preserve of particle physics, is also a major laboratory for conducting experiments in fields like biology and medicine. The electron acceleration equipment has been adapted over the past few years to create something known as the Linac Coherent Light Source (LCLS), which produces short X-ray pulses millions of times brighter than those currently created by other instruments.

The LCLS is the brightest X-ray machine in the world for the energies at which it operates -- with photon energies in the "hard X-ray" region and very high beam intensities of 10^18 watts per square centimeter. At these energies, the LCLS machine can serve as an excellent microscope for viewing matter at the scale of atoms, and biologists, chemists, and physicists have been eager to do exactly that. It also acts like a knife since it can pare electrons away from the parent atoms and molecules, even those huddling very close to the nucleus.

Becoming operational last fall, the first experimental results from the LCLS are starting to appear at scientific meetings. In San Jose, Li Fang of Western Michigan University will report on how the powerful LCLS X-rays can be used to strip electrons away from a nitrogen molecule. He says that in the extreme case, nitrogen atoms were detected from which all of the electrons had been removed. This causes the molecule to quickly dissociate. The plucked electrons, which nearby detectors can spot and measure, allow researchers to calculate the binding energy within the original molecule. In future experiments, more and more such measurements will give experimenters a more accurate assessment of large molecules, especially bio-molecules.

Presentation JFA5, "Nonlinear Processes in N2 Using LCLS Short X-Ray Pulses," by Li Fang et al. is at 9:15 a.m. Friday, May 21.

Single-cycle infrared light pulses

A pulse of light normally has many cycles of smoothly varying electric and magnetic fields. Through the use of special fibers, prisms, and optical materials, a pulse of light can be compressed down to very short temporal durations, even as short as a single cycle (only one complete wavelength of radiation).

A major reason for wanting shorter light pulses is that more data can be encoded within a signal lasting a certain interval of time. Shorter pulses would allow more data to be sent down an optical fiber, for example. Another important scientific use of very short pulses is that they can serve as a stroboscopic illumination for making movies of very short-lived phenomena, such as the movement and interactions of molecules.

Scientists at the University of Konstanz in Germany are the first to report creating a single-cycle pulse as short as 4.3 femtoseconds in the infrared region of light (which is the crucial type of light for communications applications) around 1.3 microns in wavelength. Guenther Krauss, who works with Alfred Leitenstorfer in the Department of Physics, says that another distinction of the light is that it has the highest frequency ever achieved for single-cycle pulses.

With such short light pulses, the data transmission rate for applications like the Internet might reach rates of 100 terabits per second, says Krauss. Furthermore, the femtosecond (10^-15 seconds) pulses created in the current experiments might serve as the seed for making even shorter pulses in the attosecond (10^-18 seconds) domain.

One of the biggest obstacles in microscopy and in micro-fabrication is the so-called diffraction limit. This basic law says that the resolution (or sharpness) of an image cannot be better than approximately half the wavelength of the light waves being used to make it. Similarly, when light is used to inscribe patterns on microchips -- a standard process known as lithography -- these features can't get much more narrow than about a quarter the wavelength of the light.

Now scientists at the University of Maryland have pushed this limit, achieving pattern features with a size as small as one-twentieth of the wavelength.

They do this by a clever use of two laser beams racing through a polymer solution. One beam triggers polymerization (long molecules start to link up into even longer molecules) while the other beam turns the process off. Polymerization of very narrow pillars -- much narrower than the wavelength of the light -- occurs in a tiny overlap region between the beams.

The leader of this effort, John Fourkas, says that the size of the tiny polymer structures probably represents the smallest fraction of the incoming radiation wavelength ever realized in the laboratory.

One of the structures made in the Maryland lab is a sphere-like post only 40 nanometers tall (about a million times shorter than the length of a 12-point hyphen "-"). If the polymer structures could be made conducting, then they could possibly be used in making microchips. More likely, Fourkas says, are applications in the area of biochemistry. Since the polymer structures are much smaller than typical cells, they might be used to study cell function. For example, cells could be made to "walk over" the structures, which could be used to trigger a chemical or biological response from the cell.

Additionally, the tiny polymer structures might be useful in adhesives or as channels on microfluidic chips -- little platforms on which chemical reactions can be carried out with nano-liter batches of fluids.

The sensors that allow satellites to take measurements are happiest when cold. Mechanical pumps onboard keep sensors' semiconductor elements at temperatures hundreds of degrees below zero. But these cryogenic pumps also produce noisy vibrations that interfere with the collection of data by the sensitive sensors.

Mansoor Sheik-Bahae of the University of New Mexico and colleagues are developing a technique to cool semiconductors loads that would use a vibration-free solid-state technology: laser cooling, which has traditionally been used to lower the temperature of dilute gases but can also cool transparent solids doped with rare-earth ions by kicking out energetic photons (or fluorescence up conversion). In January the group set a record by cooling a crystal down to 155 Kelvin, research published in Nature Photonics. At the upcoming CLEO meeting, Denis Seletskiy, the lead author and a senior graduate student from the group, will describe a new experiment in which the temperature of a GaAs semiconductor load was lowered down to 165 Kelvin, a useful temperature for some kinds of detectors.

"This is the only solid-state technology that can reach these temperatures, the coldest that any semiconductor has gotten without the use of cryogens and/or mechanical coolers," said Sheik-Bahae.

In addition to cutting down on vibrations, this optical refrigeration technique offers a number of other technical advantages. The laser could be guided through an optical fiber to a lightweight cooling head convenient for sensors mounted on delicate gimbals. It could also be used to selectively cool tiny areas of components much too small for other cooling technologies to selectively target.

"Our goal is to try to get colder and colder, to get to 123 Kelvin -- the NIST-defined standard for cryogenic -- and then next to 77 Kelvin, the boiling temperature of liquid nitrogen," says Sheik-Bahae. "With the right laser and the right power, we know we can get to 120 Kelvin."

"The US military is interested in applying this new research," says Sheik-Bahae. "This is quite exciting as this is a young field and more research still remains to be done in parallel to transitioning the mature components to industry. In the long term, the application of this technology to cool superconducting devices is also extremely tantalizing."

Presentation QFG1, "Laser Cooling of a Semiconductor Load to 165 K" by Denis Seletskiy et al is at 10:15 a.m. Friday, May 21.

Tunable terahertz wire laser

Terahertz (THz) radiation is one of the hottest areas of modern physics research. This is because THz light waves, or T-rays as they are sometimes called, have great potential for spectroscopy and for the scanning of objects in a homeland security setting that are opaque to infrared and visible light.

The trouble is that THz light waves -- which fall in the range of 0.3 to 10 trillion cycles per second or, equivalently, wavelengths of about 30 to 1000 microns -- are difficult to make with traditional means. Now scientists at MIT have combined several technologies to obtain a versatile source of THz light.

They start with a quantum cascade laser (QCL) device, which differs fundamentally from a traditional semiconductor laser. In most traditional lasers, light comes from the recombination of an electron with a hole (a vacancy in the surrounding semiconducting material). But in a QCL device, light comes from the transition of an electron to a succession of ever lower energy levels in a series of layers in a sandwich-style structure of thin semiconducting layers.

This type of laser has a unique property: one electron (as it moves through the layers) triggers the release of many photons. The emitted light energy of the device can be changed by altering the thickness of the layers.

Population inversion is provided over a range of energies provided by the cascaded energy levels described above with the fine energy or wavelength selection provided by the laser cavity. In the MIT approach, tuning is achieved by changing the width of the laser light beam (and hence cavity) by precisely controlling the distance between a specially designed block material and the laser. This technique is analogous to changing the pitch of a guitar string by changing its diameter. In this case, the laser waveguide is much narrower than the wavelength of the light, hence the description of this setup as a "wire" laser.

Qi Qin of MIT says their cascade laser can be tuned continuously and controllably to produce terahertz radiation over a broad range. "At present, this is the only viable mechanism to achieve broad continuous tuning in terahertz quantum-cascade lasers," said Qin.

Spectroscopy, or the comprehensive measurement of light emissions coming from an object, is the cornerstone of many scientific studies. The spectrum of a sample -- whether it comes from a star, a dilute protein solution, or the polluted air of a city street -- consists of the measured frequency of all the light absorbed or emitted by the sample, though sometimes it is difficult to accurately measure all frequencies.

Frequency can be measured quite accurately in the radio portion of the electromagnetic spectrum, where pulsations can be counted directly by electronic circuits. The "frequency comb" approach, introduced a few years ago, has revolutionized spectroscopy by allowing more accurate measurements of frequencies characteristic of infrared, visible, and ultraviolet light. The trick is to convert higher-frequency light into the lower radio frequency range, where the waves can be subjected to detailed measurement.

The word "comb" in the phrase frequency comb refers to the fact that the light being measured can be compared to a laser that emits at light at special frequencies spaced at regular intervals. The spectrum of this laser looks like a comb. This series of light frequencies serves as a sort of "ruler" against which other light signals can be compared.

Birgitta Bernhardt, a graduate student at of the Max Planck Institute for Quantum Optics in Munich, will report on a novel use of two frequency comb devices simultaneously to record broadband spectra, which speeds up the task of recording a spectrum by a factor of one million compared to the traditional Fourier transform spectroscopy. This dual-comb process has been tried before, but not previously for the important mid-infrared region ranging from 2 to 8 µm.

Mid-infrared light is important for the characterization of the structure of matter and for a number of detection problems. "The applications can be found in very different directions," said Bernhardt, "ranging from biomedicine (analysis of breath) to environmental monitoring or analytical chemistry (small traces of environmental and toxic vapors can be detected because of the high sensitivity of the measurement technique), and laboratory astrophysics."

During the CLEO/QELS Plenary and Awards Session on Monday, May 17, Gérard Mourou, professor at the École Polytechnique and director of the Institut de la Lumière Extrême at ENSTA, will present "New Physics at Extreme Intensities of Light," a discussion of the invention of the laser amplification technique known as Chirped Pulse Amplification and the field of relativistic optics, made possible by the generation of extremely high laser intensities.

Also on Monday, Douglas Simons of Gemini Observatory will present "A New Portal on the Universe -- Laser Adaptive Optics," a discussion of the new generation of enormous telescopes equipped with adaptive optics and laser beacons, which stand to transform our understanding of the cosmos and our place in it.

During the Wednesday, May 19 Plenary Session, Steven Block, a professor of physics and biology at Stanford University, will present "Single Molecule Biophysics with Optical Tweezers," focusing on recent progress in the use of optical traps to study the nanoscale properties of biological macromolecules.

Also on Wednesday, David Awschalom, director of the California Nanosystems Institute and the Center for Spintronics and Quantum Computation at the University of California, Santa Barbara, will present "Manipulating Single Spins and Coherence in Semiconductors," focusing on recent optoelectronic experiments with single electron spins in diamond that may enable fundamentally different quantum-based information technologies.

A process and method by which manipulation and orientation of a given number of directed laser beams decreases the motion of a group of atoms or molecules such that their internal thermodynamic temperatures reach near absolute zero.
The 1997 Nobel Prize in Physics was awarded to Steven Chu, Claude Cohen-Tannoudji and William D. Phillips for the development of methods to cool and trap atoms with laser light.